Hello, my company has bought (4) dell Optiplex 755 desktop computers, I have installed and readied all of them, except for one, which won't even turn the monitors on and that gives me a 2 and 4 LED diagnotic error.  According to the manual, this error is due to the graphics card.  The manual doesn't provide any solution, and I already tried reinserting the graphics card, but nothing happens.  How can I test  if the graphics card is really damaged?  Or is it the motherboard?